docs: 补充上手指引与英文 README

[gac0812]

Summary

• 补全仓库根 README.md：环境要求、Docker Compose 快速上手、Android 模拟器/真机连本机 API、语音模式 1/2、本机后端、质量检查、HTTP/WebSocket 与环境变量。
• 新增 README.en.md，与中文版对齐。
• 修正 backend/.env.example：模式 2（ASR + LLM + TTS）已经接入 /ws，不再写「尚未接入」。

Test plan

• 按中文 README 从空仓库走一遍：配 JWT、docker compose up、健康检查、frontend 安装并 npm run android
• 对照英文 README 检查章节与命令是否一致
• 确认徽章、语言切换链接可点
• 确认文档写明客户端面向 Android，且模式 2 的必填密钥与代码一致

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[fennoai]

The onboarding flow, Android networking instructions, and voice-mode environment documentation were reviewed against the current Compose, frontend, and backend contracts. The main quickstart path is actionable, but the health-check response is documented with fields the API does not return; the inline finding calls out the required correction. git diff --check passed. The targeted backend health test could not be run because uv is unavailable in the review environment.

[LUPENGHAN]

ok